The present invention relates to pulse arc welding machines. More particularly, the invention relates to a pulse arc welding machine in which a pulsive arc current (hereinafter referred to as "a pulse current") is periodically superposed on a primary welding DC current applied between a wire electrode and a base material. The primary welding DC current is hereinafter referred to as a "a base current". The molten portion of the wire electrode is formed into small droplets by an electromagnetic contraction force due to the pulse current. The molten droplets are transferred (hereinafter referred to as "spray-transferred") to the base material thus achieving the welding operation.
There has been previously disclosed a conventional pulse arc welding machine as indicated in FIG. 1. in FIG. 1, the conventional pulse arc welding machine includes a transformer 1 which employs a connection for three-phase to six-phase transformation, thyristors 201 through 206 which subject an AC input voltage to rectification and simultaneously subject the input voltage to voltage transformation by a phase control thereof, an interphase reactor 3 commonly connected between the outputs of the three thyristors 201 through 203 and the outputs of the three thyristors 204 through 206, and a main power source 4 which includes the above described transformer 1, the thyristors 201 through 206 and the interphase reactor 3 for supplying a base current. The pulse arc welding machine also includes another power source 5 which has two secondary windings of multiple turns on the transformer 1 and thyristors 601 and 602 connected respectively to the two secondary windings for superposing a pulse current on a base current. The pulse arc welding machine further includes DC reactors 701 and 702, a wire electrode 8, a wire feeding unit 9 such as a motor for feeding the wire electrode 8, a welding arc 10, and a base material (a material to be welded).
The power sources 4 and 5 are connected at first respective outputs commonly to the connecting point of the DC reactors 701 and 702 and further to the base material 11 through the reactors. The pulse arc welding machine also includes control circuits 12 and 13 for controlling the phases of the thyristors 201 through 206 as well as thyristors 601 and 602 within the power sources 4 and 5, respectively.
The operation of the conventional pulse arc welding machine thus constructed will be described.
When both a base current and a pulse current are applied from the power sources 4 and 5, respectively, between the wire electrode 8 and the base material 11 and the wire electrode 8 is simultaneously fed by the wire feeding unit 9 to the side of the base material 11, the base material 11 is welded. The control circuits 12 and 13 serve to vary the firing phases of the thyristors 201 through 206 as well as thyristors 601 and 602, thereby varying a base current I.sub.B, a pulse width .tau. and the peak value I.sub.p of the pulse current.
FIGS. 2A and 2B show examples of waveforms of the welding or arc current, i.e., the composite current of the base current and the pulse current. The frequency of the pulse current is equal to or double the fundamental frequency of the power line source. FIG. 2A shows an example of the waveform of the arc current in the case where the mean average current is small, while FIG. 2B shows an example of the waveform of the arc current in the case where the average arc current is large.
For instance, in the case where the thickness of the base material is small, a welding operation is carried out in a small average welding current range (or at a reduced wire feeding speed). More specifically, as is apparent from FIG. 2A, all of the base current I.sub.B, pulse width .tau. and accordingly peak current value I.sub.p are set small and the pulse frequency is also reduced, for example, from 120 Hz to 60 Hz in some instances.
On the other hand, in the case where the thickness of the base material is large, an average welding current (or the wire feeding speed) is increased in the welding operation. Accordingly, as is apparent from FIG. 2B, all of the base current I.sub.B, pulse width .tau. and accordingly peak current value I.sub.p are set large.
Thus, when a welding operation is carried out in a small average welding current range with the conventional pulse arc welding machine constructed as described above, as indicated in FIG. 2A, the peak current value I.sub.p is small and accordingly an electromagnetic contraction force due to the pulse current is also small. Therefore, it is difficult to transfer the molten metal in the form of small droplets 14. That is, the molten portion of the wire electrode is transferred in the form of a considerably large molten metal drop to the base material 11, as shown in FIG. 3A, as a result of which the wire electrode 8 is readily short-circuited with the base material 11 upon transfer of the wire electrode to the base material so that the molten portion of the wire tends to splatter due to the short-circuiting current thus flowing in this case.
On the other hand, when a welding operation is carried out with a large average welding current as is apparent from FIG. 2B, the pulse width .tau. and the peak current value I.sub.p is large. Thus, the quantity of heat applied to the wire per pulse period is excessively large so that the molten droplets 14 tend to droop as shown in FIG. 3C. As a result, if the arc length is set short, the wire is short-circuited with the base material resulting in splattering.
If, as shown in FIGS. 3A and 3C, a welding operation is carried out by eliminating splatter, the arc length cannot be set short, as a result of which an undercut will be created in the base material. This is a welding defect which makes it impossible to increase the welding speed.
The conventional pulse arc welding machine has a difficulty in adjustment in that, in order to obtain a short arc length as shown in FIG. 3B and a suitable molten droplet transfer state in which little undercutting occurs in the base material and the welding speed can also be increased, the pulse width .tau., peak current value I.sub.p and base current value I.sub.B must be selected within strict limits.
As described above, the conventional pulse arc welding machine has a difficulty in adjustment for obtaining a satisfactory molten droplet transfer state. Accordingly, an operator must set the constants used for the welding operation for each welding operation, and this must be done using only his own experience as a guide. This does not always result in an optimum value. Thus, the conventional pulse arc welding machine has a number of significant drawbacks.
Furthermore, even if in the conventional pulse arc welding machine the above-described welding factors are properly set, since the control circuit 13 controls the firing phases of the thyristors 601 and 602 as well as the average pulse current, the instantaneous values cannot be controlled so that the momentary molten droplet transfer state due to variations in the arc load becomes irregular and the pulse repetition frequency influences the frequency of the power source. Accordingly, the welding current range in which the optimum molten droplet transfer state is obtained is limited.
Moreover, even if in the conventional pulse arc welding machine only the pulse width, pulse frequency and wire feeding speed and the like are individually adjusted, as described above, to stabilize the welding arc at the time of starting the welding operation, no correction function is provided to compensate for variations in the arc length which are caused by various fluctuations during the welding operation. Accordingly, the conventional pulse arc welding maching further suffers from various drawbacks such as variations in the arc length, splattering occurring during due to variations in the arc length, variations in the depth of penetration, short arcing or an undercutting due to irregular base material and operator caused fluctuations frequently occurring during the welding operation.